This invention relates to a radiation detector for X-rays, .gamma.-rays and the like, and particularly to a radiation detector for use in X-ray CT (computerized tomography), positron camera and the like.
In X-ray CT, X-ray beams radiated from the X-ray source extend fanwise, pass through an object and are detected by a radiation detection system. This detection system is so constructed that 30-1,000 radiation detectors uniform in performance are placed side by side on the circular arc surrounding the X-ray source placed at the center. When the spread of X-ray fan beams is narrow, the detector may also be placed on a straight line. The X-ray source and the radiation detection system are rotated around the object. The output of each radiation detector is measured at predetermined interval of angle (for example, 1 degree) and tomography of the object is reconstructed from the output of each radiation detector.
As radiation detectors for use in X-ray CT and the like, there have hitherto been used those detectors like a xenon ionization chamber or bismuth germanate (abbreviated to BGO) combined with photomultiplier tubes. In these detectors, it was difficult to adjust the characteristics between channels so that a sufficiently clear image was difficult to be obtained with an apparatus using them. Particularly, in a detector in which BGO is combined with photomultiplier tubes, it was quite difficult to adjust the characteristics of detectors one another because of the dispersion in the characteristics of BGO single crystal used as a scintillator and the dispersion in the characteristics of photomultiplier tubes.
In order to solve this problem, some of the present inventors previously proposed a radiation detector in which phosphor particles were used as a scintillator (Japanese Utility Model Application Kokai (Laid-Open) No. 179,782/79). For the purpose of obtaining a tomography of high accuracy in a radiation detector for conventional X-ray CT, the width of the scintillator is about 1-10 mm and preferably about 1-3 mm and the length thereof is about 20 mm, for example. Accordingly, the number of phosphor particles in one radiation detector is, for example, about 300,000, though it may vary depending on the particle size. Although individual phosphor particles may possibly be slightly different from one another in characteristics, the dispersion in the characteristics as a scintillator can be made about one divided by the square root of particle number or about 0.01% by sufficiently mixing them and using the mixture as one scintillator, whereby a satisfactory result can be obtained. A radiation detector resembling to the above-mentioned one is also disclosed in Japanese Patent Application Kokai (Laid-Open) No. 90,089/79.
On the other hand, when phosphor particles are used as scintillator, detection of luminescence drops to 70-90% as compared with the case of using the single crystal. Therefore, it is preferable to use a phosphor having high radiation absorbancy and high conversion efficiency from radiation to light, such as rare earth metal acid sulfide type phosphors typified by (Gd, Pr).sub.2 O.sub.2 S and the like. However, it was revealed that there occurs in these phosphors a phenomenon of after glow, i.e. slight luminescence observed when a long period of time (1/1,000-1/100 second or longer) has passed after stopping the irradiation with X-ray. For this reason, these phosphors cannot be used in the radiation detector for some kinds of X-ray CT, or a complicated apparatus must be involved in the detection system for the purpose of eliminating the influence of after glow.
On the other hand, the following references are also known to show the state of the art: i.e. U.S. Pat. Nos. 4,031,396 and 4,071,760.